From BiblePortal Wikipedia

Hastings' Dictionary of the Bible [1]

GAI . Given as a proper name in RV [Note: Revised Version.] of   1 Samuel 17:52 ‘until thou comest to Gai,’ where AV [Note: Authorized Version.] has ‘until thou comest to the valley.’ The LXX [Note: Septuagint.] , as is noted in RVm [Note: Revised Version margin.] , has Gath , and this would suit the context.

International Standard Bible Encyclopedia [2]

gā´ı̄ ( גּיא , gay' ):In the Revised Version (British and American) of  1 Samuel 17:52 for the King James Version "valleys." the Revised Version, margin notes: "The Syriac and some editions of the Septuagint have Gath " (thus also Wellhausen, Budde, Driver, etc.).